Using T cell recognition (primarily cell-mediated lysis) we have continued to probe the complexities of HLA alloantigens. The model system in which recognition is being analyzed is analysis of the specificity of a panel of 10 previously derived cytotoxic T lymphocyte (CTL) clones. The present studies clearly indicate that the way T cells "see" DP is rather more complex than one might have predicted. Studies of these CTL clones on previously derived HLA-mutant cell lines indicate heterogeneity in specificity among the CTL clones; specifically some clones distinguish between DPw2+ DR- LCL targets which have subsequently been shown to differ in expression of DRAlpha mRNA. These data suggest that DRAlpha contributes to recognition by some "DPw2-specific" CTL clones, perhaps because they recognize: a) DRAlpha in a DPw2-restricted fashion, or b) a heterodimer formed by DRAlpha and DPw2Beta. Additional HLA-mutant LCL have been generated using the novel approach of using CTL themselves as the agent used for negative selection. More than 25 mutants have been successfully isolated. Molecular analysis of these mutants is in progress and suggests the important finding that the limited polymorphism of the DPAlpha chain (between DWw2 and DPw4) is nevertheless critical for T cell recognition.